Generally, most bicycles have a derailleur system which can force the chain to shift between different sprockets in order to achieve a variable gear ratio, so a cyclist can maintain an optimum pedaling speed while covering varied terrain. A conventional bicycle hub apparatus includes a freewheel, a sprocket unit, a derailleur, a chain, a shift lever and a cable. The sprocket unit includes two to three pieces of sprockets formed on a crank portion of a bicycle, and the chain engaged on one of the sprocket of the sprocket unit is shifted between sprockets by operating the shift lever. The freewheel includes five to ten pieces of sprockets mounted on the hub of rear bicycle wheel, and the chain engaged on one of the sprockets of freewheel is shifted between sprockets by the shift lever. The power is generated by rotating the crank and is transmitted to the rear wheel by the chain. By arranging different gear ratio, the bicycle can achieve different pedaling speeds.
Referring to FIGS. 7 and 8, the conventional bicycle hub apparatus comprises a hub body (50) with a shaft (51), and a ratchet sleeve (52) is mounted on the shaft (51) in order to secure the position of a freewheel unit (60). The freewheel unit (60) driven by the ratchet sleeve (52) is engaged with an outside periphery of the ratchet sleeve (52). A threaded segment (521) is formed at an inner periphery of the ratchet sleeve (52), and a nut (53) fastened on the threaded segment (521) is borne against the freewheel unit (60), so that the freewheel (60) is secured on the ratchet sleeve (52). Then a cyclist can shift the chain to engage with different sprockets by the shift lever in order to achieve the different pedaling speeds.
However, the conventional bicycle hub apparatus is disadvantageous because: (i) on the market, the number and the minimum size of sprockets on the freewheel (60) is limited by the size of the ratchet sleeve (52). When cycling downhill, the conventional minimum sprocket can only provide the pedaling speed for a slightly slopy terrain, and the speed of the bicycle is difficult to control when the slope of the terrain increases, so the risk of bicycle chain falling off is increased; and (ii) the acceleration of the bicycle by rotating the pedals is limited while the size of minimum sprocket is restricted, and the problem can only be solved by developing a suitable ratchet sleeve (52) which can fit the smaller sprockets. However, the solution mentioned above increases the costs of production and research and development. Therefore, there remains a need for a new and improved design for a bicycle hub apparatus to overcome the problems presented above.